This application claims priority from Ser. No. 60/60/609,039 filed Sep. 10, 2004. The present invention relates to the manufacture of carrier wafers for use in processing semiconductor wafers. In particular, the invention relates to the manufacture of carrier wafers with properties that enhance the controlled thinning of semiconductor wafers mounted on the carrier wafers.
Silicon carbide (SiC) has emerged over the last two decades as a candidate semiconductor material that offers a number of advantages over other more conventional semiconductor materials such as silicon and gallium arsenide. In particular, silicon carbide has a wide bandgap, a high breakdown electric field, a high thermal conductivity, and a high saturated electron drift velocity, and is physically extremely robust. In particular, silicon carbide has an extremely high melting point and is one of the hardest known materials in the world.
Because of its physical properties, however, silicon carbide is also relatively difficult to produce. Because silicon carbide can grow in many polytypes, it is difficult to grow into large single crystals. The high temperatures required to grow silicon carbide also make control of impurity levels (including doping) relatively difficult, and likewise raise difficulties in the production of thin films (e.g. epitaxial layers) on the material. Because of its hardness, the traditional steps of slicing and polishing semiconductor wafers are more difficult with silicon carbide. Similarly, its resistance to chemical attack make it difficult to etch in conventional fashion.
Nevertheless, based on a great deal of research and discovery in this particular field, including that carried out by the assignee of the present invention, a number of advances have been made in the growth of silicon carbide and its fabrication into useful devices. Accordingly, commercial devices are now available that incorporate silicon carbide for high-power radio frequency (RF) and microwave applications, for other high-power, high-voltage applications, and as a substrate for other useful semiconductor materials such as the Group III-nitrides.
Silicon carbide has particular advantages as a substrate for optoelectronic devices, and in particular light emitting diodes (LEDs). Since silicon carbide can be conductively doped, vertical devices (i.e. devices having anode and cathode contacts on opposite sides of the chip) can be formed. Vertical devices are compatible with most modern LED packaging equipment, which makes packaging the devices easier. In addition, silicon carbide has a small lattice mismatch with gallium nitride and other III-nitride materials.
In many new applications, LED chips are mounted in a flip-chip configuration with the epitaxial side down, permitting light to exit the device through the substrate. Flip-chip mounting is particularly advantageous for devices formed on silicon carbide substrates because of the higher index of refraction of silicon carbide as compared with that of the nitride regions of the devices in which the light is generated. However, silicon carbide can absorb some light passing through it before it is extracted, resulting in a reduced optical extraction efficiency.
Conventional silicon carbide (SiC)-based LEDs have a substrate thickness of approximately 200-500 μm. Since the SiC substrate is not a perfect conductor, the substrate increases the forward voltage (Vf) required to operate the device at a given current level. For example, the C450-CB230-E1000 LED, a typical SiC based device available from Cree, Inc. (Durham, N.C., USA) has a chip thickness of 250 +/+25 μm and a forward operating voltage of 3.5 V at 10 mA forward operating current. Reducing the forward voltage of the device would yield a proportionate reduction in power consumption since P=VI, i.e. the power consumed by the device is equal to the voltage across the device times the current through the device.
Moreover, in many new applications such as cellular phone backlighting, LED chips are packaged as surface mount devices to reduce the thickness of the optoelectronic component, which permits system designers to reduce the overall thickness of end products such as cellular phones. Accordingly, it is desirable to reduce the thickness of the LED chip to further reduce the thickness of an end product.
Accordingly, it is desirable to reduce the thickness of the SiC substrate in order to improve the operating characteristics of devices formed thereon. For example, the silicon carbide substrates may be thinned from their normal thickness down to thicknesses as low as about 100 microns or less.
Because the silicon carbide wafer must be thick enough to provide adequate mechanical stability during epitaxial growth steps, the wafer is ordinarily thinned after epitaxial deposition of the nitride layers and fabrication of devices. In order to thin the silicon carbide, the epiwafer (which comprises a growth substrate on which one or more epitaxial layers have been formed) is ordinarily mounted epi-side down on a carrier wafer such as a second silicon carbide wafer. The backside (i.e. the side opposite which the epitaxial layers are formed) of the mounted growth wafer is then thinned to the desired thickness.
However, problems with the carrier wafer may result in loss of yield from the substrate thinning process. Accordingly, there is a need for better methods of manufacturing carrier wafers, and better resulting carrier wafers.